1. Technical Field
This invention relates generally to input and output scanners, and more particularly to a scanner having a control system that compensates for scanner non-linearity electronically.
2. Background Information
A scanner includes some type of scanning means for directing a light beam to a spot on a surface to be scanned. It does so in such a way that the spot moves across the surface along a scan line in a precisely controlled manner. That enables various input and output functions such as reading a document or printing a page.
Scanner non-linearity refers to variations in spot velocity occurring as the spot moves along the scan line. It is typically caused in such systems as polygon or galvanometer laser scanner systems by system geometry or a velocity variation of the scanning means and it can affect scanner performance. A scanner having a multifaceted rotating polygon, for example, directs the light beam at a constant angular velocity. But the spot is farther from the polygon facets at the ends of the scan line than it is at the center and so spot velocity increases as the spot moves from the center toward the ends.
Some scanners compensate for the effects of such non-linearity electronically. They may include a control system with a variable frequency oscillator (VCO) that varies the rate at which the light beam is turned on and off during the course of scanning (i.e., the pixel data rate). Since the light beam turns on to begin writing a pixel, varying the rate at which it turns on and off serves to control pixel placement along the scan line. That is done according to variations in spot velocity in order to more evenly space the pixels and thereby at least partially compensate for what is sometimes referred to as pixel position distortion.
Although adequate for some applications, such compensation does not always satisfy more stringent requirements. That is because variations in spot velocity can also cause variations in pixel exposure. Sometimes referred to as pixel exposure distortion, it can result in objectionable variations in shade despite compensation for pixel position distortion. It is conceivable to vary the intensity of the light beam in proportion to spot velocity in order to offset that effect, but accurate intensity control may be difficult and expensive to achieve, particularly for direct control of a solid state or gas laser, for example, or for some type of light modulation. Thus, it is desirable to have some other way to reduce unwanted variations in shade of the type described.